This invention relates to safety interlocks for compound transmissions which include auxiliary gearboxes as utilized in heavy duty trucks and other vehicles. More particularly, the invention relates to control of movement of the shift rod of the auxiliary transmission.
Prior art auxiliary transmission devices have lacked certain desirable safety features. For example, there is often a potentially catastrophic possibility of an auxiliary transmission being shifted into gear upon failure of an air line during operation of a power take-off unit while the vehicle is stationary. Another related concern has been the possible shifting into neutral of an auxiliary transmission upon the failure of an air system while the vehicle is moving.
Several devices have been employed in the prior art which tend to alleviate at least one of these two conditions, although with some special effort being required by the operator of the vehicle. For example, a neutral interlock has been employed for assuring that the vehicle transmission will not shift out of neutral while the vehicle is stationary upon failure of air. However, the latter involves several somewhat complicated steps by the vehicle operator to effect the safety condition. In addition, extra apparatus is involved, as present structures utilize either a single 3-position air cylinder or two 2-position air cylinders to hold the transmission in neutral. What is needed is a simple, yet conveniently operated mechanism which will provide the desired combination of protections.